"FF DL-TR-65-88Part I I

S~INVESTIGATION OFSTRUCTURAL SEALING PARAMETERS

S~AND CONCEPTS FOR SPACECRAFT!I. LEA. Z-.-- 'ADE.- REPA yR

GENERAL ENVIRONMENT, AND MATERIAL PROPERTIES

Anton Hehn and Frank IwateukiIIT Research Institute

TECHNICAL REPORT AFFDL-TR-65-88, Part II

1965

Air Force Plight Dynamics LaboratoryResearch and Technology Division

Air Force Systems ConmmandWright-Patterson Air Force Base, Ohio

C L E A 1N aH 0USEIPOR PIMFRALJ SCIENT0y7C A.ND

TEctJINCAXJ I003 Miiiii

3e'o

INVESTIGATION OF jSTRUCTURAL SEALING PARAMETERS

AND CONCEPTS FOR SPACECRAFT

PART II. LEAK DETECTION - REPAIR, IGENERAL ENVIRONMENT, AND MATERIAL PROPERTIES

Anton Hehn and Frank IwatsukiIIT Research Institute

TECHNICAL REPORT AFFDL-TR-65-88, Part II

1965

Air Force Flight Dynamics LaboratoryResearch and Technology Division

Air Force Systems CommandWright-Patterson Air Force Base, Ohio

FOREWORD

The research summarized in this two-part report(Part I "Design Criteria," Part II "Leak Detection andRepair, General Environment, and Materials") was per-formed by IIT Research Institute, Chicago, Illinois,"for the Applied Mechanics Branch, Structures Division,Air Force Flight Dynamics Laboratory, Wright-Patterson

SAir Force Baaef under AF Contract AF 33(615)-2202.The research was concerned with the investigation ofparameters for efficient sealing of pressurized space-craft compartments and factors affecting the mainte-nance of efficient sealing. The Project Number is1368, "Structural Design Conceptsa" the Task Numberis 136808, "Structures for Spacecraft." Frank E.Barnett of the Air Force Flight Dynamics Laboratorywas the Project Engineer. The research was conductedfrom 4 November 1964 to 15 April 1965 by A. Hehn andF. Iwatsuki (Principal Investigators)t W. Courtney,W. DeMuth, M. Glickman, C. Gustafson, and W, Jamison.

Publication of this report does not constituteAir Force approval of the report's findings or con-clusions. It is published only for the exchange andstimulation of ideas.

RICHARD F. HOENERActing ChiefStructures Division

ABSTRACT

Part I: Design Criteria

Part II: Leak Detection - Repair, General Environment,and Material Properties

The many factors that influence the leakage of gases past aseal are discussed and analyzed, in Part I1 to apprise the designerof pressurized spacecraft compartments of the problems of achievingand maintaining a seal when extremely low leakage rates are de-sired. The nature of the leakage path is described, and the waysin which leakage occurs are categorized as interstitial, inter-facial, and permeation flow. Methods of predicting leakage forthese flow regimes are given with greatest emphasis placed on theinterfacial flow phenomenon which is characteristic of lightlyloaded, demountable seals. In the experimental phase of this pro-gram, the sealing characteristics of elastomers were studied todetermine relationships between contact stress and leakage withhardness as a parameter. It is concluded that the analyticaltechniques presented are applicable to the evaluation or designof spacecraft seals.

The information contained in Part II is related to the over-all problem of producing and maintaining a satisfactory seal butnot directly applicable to spacecraft seal design or evaluation.It is included to apprise the designer of the potential detrimentaleffects of environment on a seal. The problems of leak detectionand repair are discussed and techniques that have been investi-gated by various organizations are directly referenced. Thegeneral and induced environments to which spacecraft may be sub-jected are discussed to indicate the severity and almost endlesscombinations of environments. A brief discussion of the propertiesof rubber and plastic materials is included.

iii

CONTENTS

I. INTRODUCTION 1

II. LEAK DETECTION, LOCATION, ANDREPAIR

1. REQUIREMENTS FOR LEAK DETECTION 2AND REPAIR SYSTEMS

a. Sensitivity of Leak Detector 2

b. Time for Detection, Location 2and Repair

c. Leak Location 7

d. Warning DevLces 7

e. Reliability of System 7

f. Leak Repair 7

2. SUMMARY OF METHODS FOR DETECTION, 8LOCATION AND REPAIR OF LEAKS

a. Detection Methods 8

III. GENERAL ENVIRONMENT 28

1. TP'E NATURiAL SPACE ENVIRONMENT 28

a Pressure 28

b. RadLation 28

c. Albedo and Earth Radiation 33

d. Other Natural Radiation 34

e Meteoroids 36

2. THE INDUCED ENVIRONMENT 39

a. Kinematic Environment 39

b. Aerodynamics Heating and 40Internal Heat Sources

c. Nuclear Radiation From On-Poard 41Sources

CONTENTS •Cont.)

WV. PROPERTIES OF RUBBER AND PLASTIC SEAL 42MATERIALS

1. RUBBER 42

2. PLASTICS 45

vi

FIGURES

I Cabin Atmosphere Loss Vs. Hole Diameter 4

2 Mass Vs. Volume for Cabin Atmosphere 5

3 Pressure Decay Standard Air Composition 6

4 Total Cabin Leak Rate Vs. Time 9

5 Continuous Flow Diluent Supply Leakage 10Detection System

6 Basic Concept Ionization Gauge Leak Detector 11

7 Continuity Sensor Electrical Contact Type 12Leak Detector Locator

8 Ultrasonic Detectors 13

9 Acoustical Method of Leak Detection 14Using Delcon UltrasonLc. Translator

10 Ultrason c Translator Typical Air Leak 15Sensitivity (Orifice Size)

11 Ultrasonic Translator Typical Air Leak 16Sensitivity (Air D~scharge) Delcon Corp.

12 Location of In-Laboratory Leakage 19Measurement

13 System for Extending the Range of the 21Helium Leak Detector

14 Pressure Vs, Aititude Above Earth 29

15 Average Daytime Atmospheric Densities at 29Extremes of the Sunspot Cycle

16 Solar Spectral trradLance Outside the Earth-s 31Atmosphere at the Earth s Mean Distance fromthe Sun

17 Meteoroid Frequency as a Function of 38Meteoroid Mass

18 Typical Stress-Strailn Curves for Soft 43Vulcanized Natural Rubber

19 Hysteresis Effect of Soft Vulcanized Natural 43Rubber

Vii

TABLES

I Leak Detection and Repair 3

II Summary of Techniques for Locating 20Sources of Gas Leakage

III Summary of Leakage Measurement Techniques 22for Gases

IV Summary of Manual Repair Methods 26

V Total Maximum Surface Absorbed Dose 30Rates Vs. Time (Orbit of 200 to 300 nm)

VI Properties and Applications of Some Typical 46Rubber Compounds

VII Properties of Epoxies 48

VIII Properties of Fluorocarbons 49

IX Properties of Polyethylenes 50

VL£1

N014ENCLATURE

A = orifice area

Cd = discharge coefficient

P = cabin pressure

Ot = volume flow rate

R = gas constant

T = cabin temperaturec

V = volume

d = orifice diameter

k = constant

m = meteoroid mass

S = flux

ix

SECTION I

INTRODUCTION

This is Part II of a two part report entitled," LeakDetection - Repair, General E.nvironment, and Material Properties."The information contained herein was not included in Part Ibecause it could not be directly classified as seal analysis orseal design criteria, but nevertheless provides peripheral informa-tion which is useful to the spacecraft seal designer. The follow-ing subjects are covered in this report.

Leak detection and repair techniques

General characteristics of the natural andi~nduced environment to which spacecraft maybe exposed

Properties of rubber and plastic materialsfor seals

This information is presented primarily for the purpose ofapprising the spacecr 4esigner of the potential hazards to asealed spacecraft sa , due to the natural and self-inducedenvironment to which i± .s exposed. However, it is cautionedthat the general information which is presented must be used inproper perspective since many sealed areas will not receive directexposure, for example, to meteoroids or solar radiation. There-fore, modifications must be made to the data in accordance withthe specific spacecraft and seal design, duration of exposure,etc.

Part I discusses in more detail the problems and methods ofinterpreting and applying environmental and material propertiesdata to seal evaluation and design.

1

SECTION II

LEAK DETECTION, LOCATION, AND REPAIR

Astronauts traveling through 3pace confined within asealed pressure vessel must be provided with a life-sustainingatmosphere. Many hazards exist that could cause the loss ofthis atmospheric environment by impairment of the pressureretaining integrity of the cabin. Therefore, it is desirableto provide means for detection of leaks and pinpointing theirlocation as well as prevention of excessive loss of life sup-port and pressurizing gases. Table I summarizes the subjectspertaining to these problems which are discussed in this section.

1. REQUIREMENTS FOR LEAK DETECTION AND REPAIR SYSTEMS

a. Sensitivity of Leak Detector

Every pressurized space vehicle will have a minimumleak which cannot be eliminated without imposing unreasonablesize and weight penalties. Leakage occurs primarily past thedemountable seals which are used for items which penetrate andbreak the continuity of the pressure skin such as observationports, access doors, exit hatches, etc. Permeation of gasesthrough materials of construction add to overall leakage ratesand in most cases is small in relation to other leaks exceptwhere degradation of a material may greatly increase its perme-ability or when leaks past hermetic seals are being considered.

Examples of normal leak rates are the 300 STP cc/minleakage of the Mercury series capsules and the estimated 1000cc/min at 7-psia cabin pressure of the Apollo-type vehicle. Theleak detection system must respond to an increase in leakageabove the normal rate. Therefore, the minimum leak rate whichthe system must be capable of detecting is the total normalleakage rate or a fraction thereof depending on whether a singlesensor or multiple area-sensors are used.

Reference to Figure 1 gives an indication of the sub-stantial leakage which can tdke place through small openings.Based on the above examples of normal leakage rates, the equi-valent hole size for the minimum leaks which must be detectedcover a range of 0.003- to 0.010-inch diameter.

b. Time for Detection, Location and Repair

The time available from initiation of the leak untilrepair may be estimated by reference to Figures 2 and 3. Figure 2shows the mass of gas in the pressurized compartment versus the

2

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03 14 CO.-0 110,

u -* ~ 000

~10 0 -H.-.0 r.4 $4 V

0 1 41 _ _ _ _ _ __0_

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S 1.0/-'CC

(CD.= 0.84, Tc = 70 0 F)o.1 LI0.001 0.005 0.01 0.05 0.1 0.5 1.0

Orifice Diameter (in.)

Figure 1 Cabin Atmosphere loss vs. Hole Diameter(Ref. 1)

4

300

250

200

S00

04 10020030040

Free Cabin Volumes (ft3)

Figure 2 Mass vs. Volume for Cabin Atmosphere

5

PC - Initial Pressured - Orifice Diameter (in.)Cabin Volume - 1000 ft 3

T C- 70OF% - 0.84R - 53.35 ft-lb/lb-R-- -4.0/R-•Ac~tP/PC = a 40iTA6

VNotes lb obtain P vs. t for

any cabin volume V'- multiply t by V/1006

10 Q

Pc 10 PSIA

40P 7 PSIA

C 97,

100

2-

0.1 1 10 100 1000 10,000Time (min.)

Figure 3 Pressure Decay - Standard Air Composition(Ref. 1)

6

cabin volume. Figure 3 shows the rate of pressure decay fordifferent hole sizes which can be expected for a free cabinvolume of 1000 cubic feet. From these figures one can see thatsome leaks can be repaired at the crew's convenience, othersrequire instantaneous action, and very large leaks may causeor require decompression in which case crew safety is of primaryconsideration after which a relatively long period of time maybe available for repair.

c. Leak Location

Leaks must be pinpointed or their general locationindicated depending on the type of repair technique available.One of the difficulties in visual inspection even though holes1/16 inch and larger are readily detectable by this ;.'ethod isthe large amount of on-board equipment which will obscure theview and must be moved.

For all leaks which cannot be visually detected,other means for precisely pinpointing the leak must be devisedor a repair technique based on treatment of the entire sus-pected area may be necessary, for example, a quick settingsealant sprayed or brushed over the area.

d. Warning Devices

Two types of warning devices will be necessary sincethe size of the leak will determine the urgency and the order inwhich crew functions will be carried out. A leak large enoughto cause very rapid loss of pressure requires crew safety actionsto be utilized whereas small leaks may only require leak locationand repair. In any event the warning system should be selective,audible and visual. It must also be capable of being tested atregular intervals to insure its proper functioning.

e. Reliability of System

Since prompt detection and location and repair ofabnormal leaks is necessary to assure crew safety and continucusnission operation it is obvious that the system should have abigh degree of reliability. Simplicity in terms of number ofcomponents as well as minimum maintenance requirements willimprove reliability, although redundant systema may be necessary.

f. Leak Repair

Size of leak and its accessibility for repair will deter-nine the applicability and effectiveness of any particular repairtechnique. Therefore, it will probably be necessary to establishvell defined repair procedures and perhaps instrumentations whichvill indicate quickly the most suitable repair technique. Varia-tions in materials and characteristics of the leak path will influ-ence the selection of repair methods. For extremely large leaks

7

i

which endanger the structural intqgrity of the spacecraft,structural repair techniques such as welding and bolted orriveted patches may be required. For leakages caused by sealfailure, removal and replacement of the faulty seal must bereadily accomplished with minimum gas loss.

2. SUMMARY OF METHODS FOR DETECTION,LOCATION AND REPAIR OF LEAKS

a. Detection Methods

Various methods of detecting leaks have been proposedby various investigators (Ref. 1,2,3). Some of these aresummarized below and rore detailed descriptions can be obtainedfrom the referenced sources. In addition to in-flight methods,laboratory leak detection and measurement techniques are describedbecause of the potential applicability of some of these techniquesto spacecraft application.

(1) In-Flight Detection Methods

(a) Pressure Decay

If the cabin atmosphere consists of oxygen anda diluent gas such as nitrogen, measurement of the pressure decayof the nitrogen after its make up supply has been closed wouldenable calculation of the total leak rate. Figure 4 gives achart for calculating total leak rate from 1000 cubic foot spacefor a change in ApN2 of 5 nmn Hg. It is based upon the followingequation:

0t= 56,632 (H) (Po-P) cc/min (i)

where V = cabin volume (ft 3 )t = time (min)P =initial nitrogen partial pressure, pN 2PO= final nitrogen partial pressure

(b) Diluent Supply Rate

This method is based on the premise that theonly loss of diluent gas in the cabin will occur through leakage.Therefore, monitoring diluent supply rate will provide an indica-tion of overall cabin leak rates. One system suggested by GeneralElectric (Ref. 1) is shown in Figure 5.

(c) Vacuum Gauge Detection

Various types of low pressure measuiring g.ýuges,such as ionization gauges, could be placed in the interspace be-tween the pressure hull and heat shield to detect pressutre hIangesor presence of air molecules caused by leakage. une such concept(Ref. 1) is shown in Figure 6.

8

10,000

5000 pN 2= mmHg

39000 0 - - -

-• 2,000 -

UU

0 11•000 •I

Gi500

400

300

200

100 - - -- _ -j _1.0 2.0 3.0 4.0 5.0 8.010 20 30 40 50 100

Time (hr)

Figure 4 ToLdl Cabin Leak Rate vs. Time

Flov' control valve

-Uent

Diluent outL -(to cabin)

Orifice

Pressuretransducer

02 N2 CO2 H2Pres 2 D8

Atmosphere sensingand control unit

Figure 5 Continous Flow. Diluent SupplyLeakage Detection System (Ref. 1)

10

Heat ShieldPuncture

Leak Detector

Structure •DC-DC Converter(28 to 2000-V)

Pressure

ControlAlrPanel (Visual)

Figure 6 Basic Concept Ionization Gauge Leak Detector (Ref. 1)

(d) Discontinuity Sensor

Techniques based on an electrical conductorqii d syst.em or short circuits caused by puncture and deformationof insulated metal panels have been suggested. One method issho•Wn schematically in Figure 7.

1]

;•AI sheet•

MylarElectrical(insulation) contact

Before puncture After punctureBasic Concept

Detector "A" Detector "B" Detector "N"d- - T"-

I I

Jare "N7 - +28 VDC

Lighbt Switch

Light switch' area "A"l

Control panel

Electrical Schematic

Figure 7 Continuity Sensor Electrical ContactType Leak Detector - Locator (Ref. 1)

12

UltrasonicRadiator Pickup

Flaw

(a) Continuous Wave Detector

Pulse Echo

(b) Reflected Wave Detector

Figure 8 Ultrasonic Detectors

13

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10. 20. 30. 6(0,60•B1D 2 3 4 6 810 20 3D 40 60 80 100

Detection Distance (ft)

Figure 11 Ultrasonic Translator Typical Air Leak Sensitivity(Air Discharge) Delcon Corp. (Ref. 9)

16

(2) Laboratory Leak Detection Methods

Method 1. Spark Coil Over Outside of System

A high potential electrode is traversed over theoutside of a system or component located within a vacuumenvironment. The leak is found by spark passing through theleakage flow or by a change in discharge color with materialssuch as ether, alcohol, or carbon dioxide.

Method 2. Pressure Rise Measurement

By monitoring the change in pressure in an isolatedvolume, the leakage rate may be calibrated.

Method 3. Detection of Tracer Gas with ThermalConductivity or Ionization Gauge

Change in pressure within the rystem may be monitoredusing the thermocouple, Pirani or ionization gauge. A tracer gasapplied to the exterior enters the leak and changes the thermalconductance or ionization current flow of the gas which was initi-ally in the system. A rise in indicated pressure shows thepresence of a leak. Similar techniques employing sealing sub-stances on the outside of a system may be used also. In this casesealers such as water, lacquer, Glyptal, acetone, or ether clogthe leak and cause a decrease in indicated pressure. Some of thesealers such as ether, provide a temporary seal while lacquer ispermanent.

Method 4. Radioactivity Measurement

A radioactive material, such as Krypton 85, is allowedto enter a leak from the outside. Usually the external pressure ishigh. Leakage into the system is permitted for a given periodafter which the component is removed from the radioactive environ-ment. The amount of radioactive material which has entered thecomponent is measured by a scintillation counter.

Method 5. Halide Torch

A halide containing gas such as Freon is introducedinto the system under pressure. A propane torch is used to heata copper plate and the Plate is traversed over the system. A leakis detected when the flame turns bright green.

Method 6. Hydrogen Leak Detector

Hydrogen tracer gas may be detected using an ioniza-tion gauge and a heated paladium diaphragm. Changes in gaugeoutput show the amount of leakage.

17

Method 7. Halogen Leak Detector

Halide containing gases, when in contact with ahot platinum wire, change the positive ion emission of theplatinum. Electrode current is measured, giving an indicationof leakage.

Method 8. Helium Leak Detector

Leakage gases containing helium are ionized and theintensity of ion current produced is used to indicate the heliumflow rate. Helium is separated from other gases present by arelative acceleration process. Since the mass of helium is low,it separates readily from other gases. Because of this property,helium is most commonly used with leak detectors which operateon this principle.

Method 9. Bubble Inspection

Bubble detection may be a':complished by applyingpressure within the system and applying soap or other wettingfluid on the exterior. Bubbles are observed visually.

Method 10. Bubble Inspection (Immersed)

This method is similar to Method 9. In this casethe component is submerged in a liquid, usually water. Bubblesare observed visually.

Method 11. Positive Displacement Devices

Leakage flow may be measured by positive displace-ment techniques providing it can be collected. The most commondevices are:

a) Vane displacement meters. Instruments of thistype are used to measure relatively large leakage rates. Therotational displacement of the device may be used to indicatetotal volume flow or flow rate.

b) Bellows displacement meters. The displacement ofa pleated bellows or diaphragm may be used to measure volumechange or, with suitable instrumentation, flow rate.

c) Glass capillary tubes. Very small diameterglass tubes containing a nonwetting fluid bubble may be employedto measure small leakage. The movement of the bubble must bevisually observed.

Method 12. Rotameters

A tapered tube containing a flow is commonly used tomeasure large flow rates. The position of the float in the tubeindicates a flow rate. Since the tube is tapered, the flow areaabout the float varies with position.

Method 13. Acoustic Detectors

Audio detection of leakage flow may be determinedunder certain conditions of flow. It is usually applied wherelarge leaks are involved and choked or turbulent flow occurs.

Method 14. Chemical Reaction Methods

Coating of components with thin films subject tochemical change with the leaking fluid may be employed If thechemical change produces a discoloration of the film and is notdetrimental to the component.

Summary of In-Laboratory Leakage DetectionInstruments and Techniques

Source Locating Techniques

The source of gas flow may be readily located. Themost sensitive methods employ the halogen or helium leak detectorwith a probe. A disadvantage is that a tracer gas is required.The bubble technique is satisfactory if sufficiently sensitive(10-4 atm cc/sec), the component is not too large, and the geo-metry is such that the leak is visually observable. Table II showsa summary of the methods which may be employed for gases.

Quantitative Leakage Measurements

Quantitative leakage measurements may be made atseveral locations in a system or component. Figure 12 showsschematically the principal locations of leakage measurements.

Leakage FlowSA j B C -•

System or Component

being Checked

Figure 12 Location of In-Laboratory LeakageMeasurement

C)

Table II

Summary of Techniques for Locating

Sources of Gas Leakage

Method Remarks

Spark coil Difficult to apply to largeobjects. Requires insula-tion and vacuum environment.

Change in pressure Requires trial and errorapplication of sealants.

Halide torch

Halogen leak detector Probe attachment required.

Helium leak detector Probe attachment required.

Bubble inspection (soap) Bubble formation is timeBubble inspection dependent and varies with(immersion) leak rate. Satisfactory

for leak rates of 10-4atm-cc/sec.

Sonic detectors Applicable to large leakrates or under special flowconditions. Influenced bybackground noise.

,Chemical coating Time dependent and limited

by gas and coating material.

20

I!

By using various techniques7 some. instruments may beemployed to measure leakage by one or more of the followingmethods:

Method

A Leakage in

B Leakage in minus leakage outand measured within

C Leakage out

Some of these techniques are capable of measuring total leakageflow while others are capable of only sampling the total flow.The latter method is demonstrated by probing types of instruments.Table 3 tabulates the available instruments and their approximateranges of sensitivity. The range of the helium leak detector maybe extended up to 10-3 cc/sec by applying it in a system as shownon Figure 13

Leak

~v~c) Detection

Leakage Flow tv)¢a

00-' - Forepumnp --

'"v(d)

Figure 13 System for Extending the Rangeof the Helium Leak Detector

In the system shown, the pressure at point (a) is maintainedconstant. If the leak detector is calibrated, a calibrated leakmay be placed .at point (b). The ratio of flow division may beadjusted by varying throttling valves (c) and (d). The measuringsystem is then connected to the component being evaluated.

21

Table IIX

Sumary of Leakage Measurement TechnLque. for (eaes

LDekag. Rate Sensitivityatm' cc/$ea

Vacuum system applications.as 30 Mexinum lakang. flow depends

upon pumping Syastm, pC48-sure and temperature.

Accuracy dependaent prosure measurements.. ==emp

ab RD tibia to temperature changes.Constant leaI ge presourecannot be mintained.

3 Same as 2a above.

Limited by the MISS of the

4 no component and influence ofradioactivity.

6 RD

7a cc Using probe attach mnt.

?b lID Installed within Compoment

Ba cc Using probe method.

lb CD

Bubble rate of formation or

10 CD) bubble aile neoossary foraccurate measutement.

ADIla or

CDAD Depends on bellow is,..

A-ur Depends on tube diameter... I- _ E.- -. -..

12 CD

Note 1 - Techniqjue Identifications A - leakage in,U - (Leakago In - laakIrle out), C - Leakage out,L) - Total flow nedsurenent., 3 - Partial flow measurement.

22

Bubble measurement techniques are probably the most widely usedfor general inspection. When accurate and reproducible quanti-tative measurements are required, other methods must be employed.Caution must be used in employing fluids which readily wet theleakage path. Surface tension effects occur, limiting leakageto levels well below the actual or operational level.

Most of the leakage instruments provide a readout interms of leakage rate. Exceptions are methods 2, 3, and llc.When using the pressure change method for gases, the sensitivitymay be calculated from:

Q = AvAP/At (2)

whereAV = container volumeAP = pressure changeAt = time increment

Temperature changes influence the accuracy of this method, de-pending upon the leakage rate and container volume. Anotherdisadvantage is that a pressure change must occur. Thus,leakage information may not be readily correlated with otherdata.

(3) Leak Location Methods

Many of the leak detection techniques discussedabove are applicable to leak location. In addition, visual aidtechniques using fluorescent or colored materials and magneticparticles may be useful particularly if access to both surfacesof the pressure wall is possible. These methods are brieflydiscussed below.

(a) Fluorescent Penetrant Inspection

This method is used in industry for inspec-tion of metallic parts for surface cracks or pores. It is asensitive way of detecting minute cracks invisible to the nakedeye. The procedure used is to apply a fluorescent liquid pene-trant over the surface to be inspected, allowing it to enter anycracks or pores. The excess penetrant is removed and the areais viewed under ultraviolet (black)light. The penetrant seepingback from the cracks fluoresces and shows the defects in theinspection zone. A powdery developer which acts as a blotter isalso used to aid the penetrant to emerge from the cracks.

In order for this method to be applicable,the general location of the leak must be known and the surfacecleaned of particles that could keep and hold the penetrant. Thepenetrant must be applied by a brush-type applicator complete withpenetrant reservoir in a thin, even coat. Since a water spray

23

is not applicable to remove the excess penetrant, it can beremoved with a wet or solvent soaked sponge. A developing powderin a colloidal suspension should be applied to the surface sincea plain powder would contaminate the cabin with dust. Exclusionof normal light is necessary for viewing under an ultravioletlight.

The difficulties encountered with thismethod are the penetrant leaking through the crack or hole thatis the subject for location. This would be enhanced by the pres-sure differential. Some penetrant will also volatilize. Thus,there will be less penetrant available to re-emerge from thecrack under the action of the developer. Because of this, thetime between excess penetrant removal and addition of the de-veloper and drying should be as short as practical which maynecessitate inspecting small areas at a time.

(b) Colored Penetrant Inspection

This method of inspection utilizes a coloredpenetrant, usually red, in a method similar to the fluorescentpenetrant technique. In this case, however, ultraviolet lightis not used as the penetrant. This method is not quite as sensi-tive because the indication is not as brilliant.

(c) Magnetic Particle Inspection or"Magnaflux" Inspection

In this method, a magnetic field is estab-lished in the test object, magnetic particles are then appliedto the surface, and the surface is examined for accumulations ofthese particles. Since a magnetic material is required for thismethod to work, a thin magnetic membrane must be bonded to anynon-magnetic structure.

(4) Leak Repair Techniques

The types of repairs required can be divided intothree categories, according to the type of leak. These are(1) repairs of large punctures of the cabin wall, (2) repairs forsmall punctures and cracks, and (3) repairs of seal leakage. Themethods for repairing these different types of leaks need not bethe same, but it would be desirable, in order to reduce complex-ity, to utilize the same materials or methods to repair all leaks.

(a) Large Holes

1) Plug patch

* Self brazing plugo Plug and sealant

24

2) Flat patch

"* Welded patch"* Adhesive bonded patch"* Mechanically fastened patch

(rivets, bolts)

(b) Small Holes and Cracks

1) Sealant

"* Brush type"* Spray type"* Putty type

(c) Seal Failure

1) Seal replacement

2) Seal repair

Table IV taken from Reference 1 is a summary evaluation of somemanual repair techniques which are applicable to sealing ofpunctures and cracks.

(d) Self-Sealing Methods

A self-sealing layet bonded to the pressurewall provides an automatic repair method for smaller punctures.The advantages of such a method are self-evident. It eliminatesthe need to detect, locate, and subsequently repair the smallpunctures. Repair will be almost instantaneous, and very littleatmosphere will be lost. However, in the design of a self-sealing layer: there will be a maximum size puncture that can besustained and sealed. Punctures above this size will createcabin leakage. If provisions are to be made for the repair ofthese larger punctures. a leak detection and location system ofthe type discussed previously will be required. Therefore, thereal benefit of a self-sealing cabin wall is the automaticity ofrepair. That is for many punctures, the crew will not be re-quired to locate the leak. or repair it.

Several concepts for self-sealing of pres-surized structures exist. These design concepts contain some ofthe following self-sealant materials.

Mechanical Self.-Sealants Chemical Self-Sealants

Flexible fodms Latex sealantsMicroballooms Swelling sealantsCuted elastomeric sealants Chemical reactant sealants

25

Table IV

Summmary of Manual Repair Methods

Repair Method _ Application Remarks

Patch, metal mechan- For repair of large 1. Only method appli-ically fastened, punctures where cable to odd sizesealant backed cabin is pressur- punctures, tight

ized and rework is corners - allallowed situations short

of repair

Patch, metal, bonded 2. Hard to fit con-toured wall

Patch, elastomeric 3. Same as 2bonded

Plug, elastomeric or 4. Cannot be used inmetal bonded arid every casesealed

Plug, metal self- 5. Same as 4brazing

Putty sealant - Small punctures and 6. Forms tough, dura-curable two-part cracks - cabin pres- ble sealcompound surized, no rework

of damaged area

Putty sealant - 7. Same as 1 - epoxycurable two-part type has unexcelledcompound strength and adhe-

sion

Liquid type - cures 8. Good for only theto form film smallest holes and

cracks

Patch, elastomeric, 9. Reliability andadhesive sealer application lowbacked because of inter-

ference from pro-truding edges ofpuncture

26

Table IV (Cont.)

Repair Method Application Remarks

Patch, hollow, adhe- Small punctures and 10. Tight corners prob-sive sealer backed cracks - cabin pres- lems

surized, no reworkof damaged area

Patch, metal de- 11. Tight corners prob-pressed center, lemsgasket adhesivesealer

Plastic sheet adhe- 12. Good for only small-sive backed est punctures and

cracks

Plug, elastomeric 13. Jagged puncturesand sealant problems

Putty sealant - Repair of sealscurable one-partcompound

Putty sealant -curable two-partcompound

Liquid brush, rollor spray-on typecure to form film

27

SECTION III

GENERAL ENVIRONMENT

The purpose of this section is to summarize currentlyavailable information on the space environment and to identifyand quantify these environmental factors which affect the se-lection of a seal material or configuration for a particularspace application.

1. THE NATURAL SPACE ENVIRONMENT

The term "natural" is used here to define those environ-mental properties which exist whether a space vehicle is thereor not. Those environmental effects caused by the presence ofthe space vehicle are termed "induced" and are discussed inSection 111-2 following.

a. Pressure

The pressure of the atmosphere at a given altitudeabove the earth is not invariant but subject to a number ofdynamic effects. However, good averages will suffice for thisprogram. Figure 14 (Ref. 1) shows the average atmospheric pres-sure with respect to altitude up to about 400 miles. The pres-sure decreases with altitude from the average of 760 mm Hg atsea level to about 10-6 mm Hg at 125 miles. Reference 2 lists3°6 x 10-9 torr as the most severe vacuum likely to be encoun-tered in a 200- to 300-nautical mile orbit about the eartNAbove 4,000 miles altitude, the pressure is less than 10-4 mm Hg;and in interplanetary space, the pressure is thought to be 10-16mn Hg, which corresponds to a density of 4 molecules/cc.

In the orbital range of about 100- to 400-miles alti-tude, pressure estimates given by different authors differ byabout one order of magnitude. In addition to the difficulties inmeasuring pressure (or density) in this thin atmosphere, thesunspot cycle affects the density (pressure) measurably. Thisis illustrated graphically in Figure 15 (Ref. 3).

b. Radiation

Because of the shielding effect afforded by the earth'satmosphere, much of the radiation existing beyond the atmospherecannot be measured on the surface of the earth. The radiationcomponents of the space environment are sundry, complex, and insome cases, unknown. Theoretical models have been proposed forsome types of space radiation, and radiation data have been ob-tained using rockets and other experimental means. The datapresented here is based on the best presently available informa-tion; new data, particularly that being collected by the Explorer

28

500

400

300

ro

.4-

"• 200

I)

-10 - 6 4 -

10-I0 10-8 10-6 10..4 10.2 1 102 10

Pressuie (nui-Hg)

Fiqure 14 Pressure vs. Altitude Above Earth

2500 1500

200010-

0 -2oo4J .,-4

,-4: 500

1020 1015 1 0

Density (gm/cm )

Figure 15 Average DaytimeAtmospheric Densities atExtremes of the Sunspot Cycle

29

series of space probes, can change these values considerably insome cases. The values presented are, in general, for nearearth space; little data is available for deep space.

A summary of the various space nuclear radiationcomponents encountered in a 200- to 300-nautical mile orbit arepresented in Table V (Ref. 2). These data are an integratedyearly estimate, and are thus influenced strongly by short peri-ods of high activity. So-called normal levels would be muchlower. The values are based on the incident surface energyreceived on each square centimeter of space vehicle area. Anexplanation of the terms in the table is given in the ensuingdiscussion.

Table V

Total Maximum Surface AbsorbedDose Rates vs. Time

(Orbit of 200 to 300 nm)

Orbital Inclination28.50 900

Source (rad/vr) (rad/yr)

Solar flare events 30.0 450

Electron belt 3.0 x 106 106

Proton belt 0.3 x 106 0.20 x 106

Miscellaneous radiation 145.0 108

Total 3.3 x 106 1.01 x 108

(1) Solar

The bulk of the energy in the solar electro-magnetic spectrum lies 4between the wavelength limits of 0.3 and4.Op (I•t = 10- 4 cm = 10 ) , with approximately one percent ofthe energy lying beyond these limits. Figure 16 (Ref. 3) showsthe distribution of energy in the solar radiation incident onthe earth's upper atmosphere at the mean distance from the sun.The solar constant is the total solar irradiance; it is equalto the are• under the curve of Figure 16, and has the value of0.140 w/cm (443 Btu/ft 4 -hr).

(2) Thermal

The visible and infrared (the ultraviolet ncKincluded) portion of the solar spectriun is well approxtmat'- inspectral quality by the radiation emitted by a 6000 0 K blac7-body. This will be felt by a space vehicle as solar thcrmal

30

I

4E

ftCt

C

tC

0.25 t

0.20

0.15

14 0.11-4

r-4

4 0.05

0 L0 0.5 1.0 1.5 2.0

Wavelength (I)

Figure 16 Solar Spectral Irradiance Outside the Earth'sAtmosphere at the Earth's Mean Distance fromthe Sxn

31

radiation. The product of the vehicle's surface absorptivity(to 6000 0K blackbody radiation) and the solar constant will beequal to the solar thermal input to the side of the vehiclefacing the sun. The portion of the space vehicle not facingthe sun or the earth will radiate thermal energy as a functionof its local temperature and its surface emissivity at thattemperature, to deep space, which acts as a blackbody at about30K. The net thermal radiation (difference between input andoutput) at a given space location determines the local spacevehicle surface temperature whether the heat flow is into thevehicle or out of it. Space vehicle temperatures are expectedto lie in the range between -10O0F and 3000F. In the interiorof the vehicle, thermal control systems will hold temperaturesto within a few degrees of a preselected temperature based onhuman or equipment requirements.

(3) Ultraviolet

The ultraviolet radiation of the near-earthspace environment will be higher during solar flare activitythan during quiescent sun periods. Since the solar flare eventsare known to be cyclical in character, the exact ultravioletintensities incident on space vehicles will be a function of theoperational time period of the vehicle.

Ultraviolet radiation has wavelengths from 0.01to 0.4t. Since the solar energy less than 0.4jj is about 9 per-cent of the total solar constant, the integrated intensity fromthe ultraviolet component is 12.6 milliwatts/cm2 for the "quietsun.4"

The solar spectrum below 0.3L must be obtainedfrom rocket observations as virtually none of this radiationpenetrates the earth's atmosphere. Below 0. 2 2p., the solarspectral irradiance curve is well approximated by the radiationwhich would be received from a blackbody source the same sizeas the sun and having a temperature of 45000 K. In addition tothis continuum radiation, there are a number of emission lineswhich contribute only a small amount of energy compared to thecontinuum above about 0.14j±.

(4) Solar X-Rays

Although the major portion of the electromagneticradiation from the sun is not ionizing in nature, a very smallportion (about 0.1 percent of the solar constant) lies in thesoft ,-ray region of a few kilovolts. On this basis, the surfacedose rate is estimated to be about 106 roentgen/hr and higherafter solar flares. Since this X-ray energy is absorbed stronglyby materials, the dose rate in the interior of the space vehiclewill be negligible.

32

(5) Solar Flares

The previous discussion has been concerned withthe electromagnetic radiation emanating from the sun. The sunalso gives off very high-energy particles whoza characteristicsare very similar to cosmic rays. The flux of these particles,observable near the earth, is highest about an hour after thefirst detectable signs of intense solar surface activity (calleda solar flare).

Some data exists on high-energy electrons andelectromagnetic radiation associated with solar flares, but themajority of the activity is due to intense fluxes of energeticprotons (hydrogen nuclei). The proton flux arrives on the sunlithemisphere of the earth and generally lasts between 10 and 100hours with an intensity decay dependence with time of about t

Because of the magnetic field of the earth, theprotons (being charged particles) are deflected away from theequatorial regions and are most intense in the polar cap regions.Most of the solar flare proton events observed have involvednon-relativistic protons which are stopped in the earth's atmos-phere, but relativistic protons have been observed at the earthssurface. Fluxes may be as high as 104 protons/cm2-sec for largeevents. Average dose rates may range from 1 to 100 roentgen/hrand the total dose would range from 10 to 103 roetgens.

(6) Solar Wind

The solar wind is regarded as a continuous fluxof particles (protons and/or electrons) from the sun in itsnormal quiescent state. The distribution of solar wind particlesis believed to obey an inverse square law with the sun actingapproximately as a point source. This distribution would beperturbed by the earth's magnetic field. The solar wind wouldbe prevalent at altitudes of several earth radii or greater, andthus would not be felt by an orbiting space vehicle.

c. Albedo and Earth Radiation

Temperatures of near-earth satellites are determinedto a large extent by the thermal radiation interchange betweenthe earth and the satellite and by the surface absorptance-emittance characteristics of the vehicle. Heat is radiated tothe spacecraft directly from the earth's surface and reflected(albedo) from the sun off the earth's atmosphere. The thermalradiation from earth to the space vehicle is about twice asgreat with clear skies, but the albedo radiation is about 3 timesas great with cloudy skies as with clear skies. The relativecloudiness affects thermal radiation more than the other radia-tion components.

33

The albedo (reflected solar) radiation containsseveral other important components in addition to thermal radia-tion, while the earth radiation consists entirely of infraredenergy (thermal), nearly all at wavelengths longer than 4.0g.The other components of albedo radiation include ultraviolet,neutron, proton, and bremsstrahlung flux. The spectral inten-sity of albedo electromagnetic radiation is highest in therange between 0.29 and 0. 4p., the ultraviolet region. Particu-late albedo radiation is discussed below.

(1) Albedo Neutrons

The interaction between primary cosmic rays andthe atmosphere produces neutrons, some of which recoil away fromthe earth. An upper limit of 0.8 neutrons/cm -sec at 200-nm al-titude has been established with an energy peak in the 1-10 evregion. This produces a negligible dose.

(2) Albedo Protons

In addition to recoil neutrons, protons are alsoproduced by cosmic ray primaries. At 200 nn, there exists a fluxof about one proton/cm2 -sec between energies of 1 to 10 mev.This produces a negligible dose.

(3) Bremsstrahlung Flux

Electron interaction with the atmosphere producesa low energy (20 kev) gamma radiation. The expected flux at200 nm is given by various references to be about 104 photons/cm2 -sec. This yields an expected maxinum surface dose rate of 10.7millirad/hr, or 93.6 rad/yr. This Bremsstrahlung radiation isreadily absorbed by the spacecraft walls where it is subsequentlydissipated as heat.

d. Other Natural Radiation

The other types of radiation to be described aremostly particulate in nature and are generally highly penetrating.Some of these phenomena are highly localized and can be avoidedby not entering the zones where they are prevalent.

(1) Cosmic Radiation

Except for such localized effects as the deflec-tions caused by the earth's magnetic field, the distribution ofprimary cosmic particles in space is essentially isotropic anduniform. The normal primary cosmic particle flux in free spaceis about 2 particles/cm2 -sec, and its composition can be describedas abcut 90 percent protons, 10 percent alpha particles (heliumions), plus very small fractions of heavier ions having atomicnumbers up to about 30. These cosmic primaries are moving withrelativistic or near-relativistic velociti?8; their energyspectrum is very high, mostly from 1 to 10 bev.

34

Most of the cosmic rays impinging on a spacevehicle will traverse with very little interaction or loss ofenergy. However, these are secondaries formed by cascadeshower phenomena and by high-energy nuclear interactions suchas spallation and star formation. The effectiye ionizationdose rate due to cosmic primaries is about 10- roentgen/hr,and the approximate effective dose due to secondaries producedin a space vehicle or in the atmosphere is about 10-3 roentgen/hr.

The earth's magnetic field causes considerabledeviation in cosmic ray flux, so that in an earth orbit the dosewill vary from about 8 to 50 rad/yr, depending on altitude andorbital inclination. Shielding against cosmic ray primaries isnearly impossible due to the high penetration of the primariesand the secondary radiation, but the dose rates are low enoughto be tolerated by most materials except for very long periodsof time.

(2) Trapped Radiation (Van Allen Belts)

The spatial distribution of the Van Alien beltsmay be visualized as a torroidal configuration encircling theearth about its geomagnetic equator. The Van Allen belts canbe considered as a proton belt. centered around 2,000 milesabove the magnetic equator, and an electron belt, centeredaround 10,000 miles; these belts intermingle at lower altitudes.

The maximum proton flux at the heart of theproton belt is estimated to be 3 x 104 protons/cm2 -sec. Thisflux corresponds to a dose rate of about 30 roentgens/hr.Vehicle interior dose rates for protons is not much differentfrom the surface dose rate although present data are not ade-quate for an accurate comparison.

The electron flux at the most intense portionof the electron belt is 2 x 108 electrons/cm2 -sec, and the cor-responding surface dose rate is about 105 roentgen/hr. Themaximum vehicle interior dose rate in this outer belt regionranges from 10 to 100 roentgen/hr.

While the proton belt is fairly stable in posi-tion (with respect to te earth) and flux rates, the electronbelt is suspected by many investigators to shift radically influx level and in energy spectrum with solar activity and asso-ciated phenomena such as magnetic field perturbations. Estimatesof the total yearly surface dose in a 200- to 300-nm or.-t aregiven in Table V,

35

(3) Auroral Radiation

Auroral radiation fluxes are found most fre-quently between 65 and 70 degrees north and south magneticlatitudes. The loci of the auroral radiation zones may bevisualized as crown-like configurations sitting over the polarcaps. Polar orbits of earth satellites include four traversesthrough the auroral region per orbit, while equatorial orbitsdo not include the auroral regions. The occurrence of auroralradiation is much more sporadic than the Van Allen radiation.

Electron fluxes measured during auroral stormsare known to be as high as 1011 electrons/cm2 -sec. Correspondingto a surface dose rate of 108 roentgen/hr. These maximum fluxesare believed to correspond to sharp spikes and sheets of auroralradiation and are not believed to pervade the entire region.Because of the sporadic nature and spatial distribution ofaurora, it is generally agreed (Ref. 2 and 3) that the statis-tical average surface dose rate per year would be about 107 to108 roentgen. The low-energy (mostly <50 kev) electron pri-maries are not expected to penetrate the typical vehicle skin.Secondary bremsstrahlung radiation reaching vehicle interiorsis estimated at about 10 to 15 roentgen/hr. The statisticaldose rate received over several days orbiting will be dominatedby the dose received during active auroral displays.

Auroral protons are much lower in flux than theauroral electrons. The flux is about 105 protons/cm2 -sec withenergies ranging up to about 650 kev. The corresponding surfacedose rate is about 500 roentgen/hr in active aurora, and theinternal dose due to protons is neglijible.

Shielding vehicle interiors against auroralradiationboth primary and bremsstrahlung, is relatively simpleusing multiple element or laminated materials, but vehicle ex-terior surfaces can be damaged considerably from long exposuresto the high electron fluxes.

e. Meteoroids

The discussion presented in this part on meteoroidsis abstracted almost entirely from Reference 4. It is a goodsummary of and in close agreement with the material presentedin References 1, 27 3, and 5. More detailed descriptions ofthe meteoroid environment are presented in Reference 1 and 5.The ensuing discussion delineates the "cloud" of meteoroidssurrounding the earth to about the cis-lunar distance or 60earth radii. Flux rates for meteoroids in interplanetary spaceare considered to be much lower since meteoroids become trappedin the earth's gravitational field.

36

(1) Flux Rate

Observations on the meteoroid flux about theearth have been made for larger meteoroids through the utiliza-tion of various photographic and radar techniques from ground-based stations. Measurements of the flux rate of smallermeteoroids have been made primarily through the use of variousmeasuring devices carried by rockets and satellites. The re-duction of all this raw observation data to a useful form isbased upon certain assumptions of meteoroid physical charac-teristics (e.g., density, luminous efficiency, etc.). In thecase of the smaller meteoroids, data reduction is complicatedby impact interactions with other satellite instrumentation.The uncertainties involved in meteoroid observations have re-sulted in estimates of meteoroid influx rates which differ byorders of magnitude. The two curves shown in Figure 17 areintended to bracket the average actual meteoroid distributionin space. The actual distribution is, however, quite nebuloussince it is believed to vary with both time and with locationin the solar system. The various proposed meteoroid influxrate relationships, including those shown, may be expressed byan equation of the form

P = K/m (3)

where 0 = flux in particles/unit area/unit time, havinga mass greater than m

K = a constantm = meteoroid mass

It is generally agreed, as the above relation-ship indicates that the number of meteoroids increases as theirmass decreases. From the standpoint of providing meteoroid pro-tection capabilities in spacecraft designs, the large meteoroidsand asteroids may be neglected because of 1) their rarity, and2) the impracticality of attempting to provide protection againstsuch an impact force. However, compartmentalization can isolateand minimize large meteoroid damage. The very small meteoroids(micron size) may cause erosion of surface materials. It isestimated that the intermediate sizes (10-3 to 1 centimeter) willpresent the greatest hazards from skin penetrations.

(2) Velocity

There is general agreement that the absolutevelocity range of meteoroids is between about 11 and 73 km/sec.The lower velocity limit is determined by the velocity a par-ticle would attain if it fell from a great distance under onlythe influence of the earth's gravitational field. The uppervelocity limit is based upon the assumption that the earthruns head-on into a particle which is in retrograde orbitaround the sun and is determined by adding the earth's orbitalspeed to the maximum orbital speed of this retrograde particle

37

10 4 I I I I .1 I

102

100 Pessimistic AverageMeteoroid Flux

10-3 x

10-4

:. \10-8 _ -.

lo -optimistic Averagei

Meteoroid Flux

10-14 -10- 16 x

10-16 __

10-18 -% -

10- 1 2 0-10 10-8 10- 6 10-4 10- 2 100 102 104 106

Meteoroid Mass (gin)

Figure 17 Meteoroid Frequency as a Function of Meteoroid Mass(Ref. 4)

38

at the earth's radial distance from the sun. It is furthergenerally agreed that the velocities of the smaller mass par-ticles probably lie near the lower velocity limit. Whipple's1961 estimate (assuming the mass of a zero visual-magnitudemeteor to be 2.0 gm) proposes that an average velocity of 15km/sec be used for particles up to l0-7-7 gm.

2. THE INDUCED ENVIRONMENT

Induced environments are created by the vehicle systemitself, from internal energy sources, or by reaction with thenatural environment. Specific definition of induced environ-ment is thus highly dependent on particular characteristics ofeach vehicle system. Careful design of space vehicle systemswill assure that the effects of the anticipated induced environ-ments will be within tolerable limits, particularly on mannedmissions. The following discussion, abstracted mostly fromReference 6, is necessarily general in nature and qualitativerather than quantitative, although illustrative values are givenas an indication of the order of magnitude of the effects.

a. Kinematic Environment

Kinematic environment is that which is produced byvirtue of motion in a mechanical system. Although this subjectis treated here under the separate headings of acceleration,shock, and vibration, these variables do not occur independentlyin a real vehicle system.

(1) Sustained Acceleration

Typically, high-force sustained accelerationsare encountered for boost and re-entry phases of space flight.Duration of exposure to sustained acceleration during boostvaries from a few seconds to about 200 seconds per stage.Typical sustained launch and boost accelerations predictablefor the next decade aie: 8g for large vElicles (for the longesttimes); 20g for medium sized; and 50 to 100g for small vehicles(for the shortest times). Manned vehicles will be limited topeaks of 8 to 12g for short durations, and less for longerdurations.

Maximum acceleration (deceleration) attainedduring atmospheric re-entry is dependent only on path angle,initial velocity, and characteristics of the atmosphere. Forinstance, vehicles entering the atmosphere vertically from spacecan encounter accelerations of 300g, while vehicles entering at15 degrees below the local horizon will experience 8.5g decelera-tion.

The condition of "zero gravity" is a ratherspecial case of sustained acceleration and is generally associ-ated with an orbiting vehicle whose radial acceleration justbalances the earth's gravitational force, or with a vehicletravelling in deep space.

39

(2) Mechanical Shock

Mechanical shock is not easily distinguishedmathematically from vibration or accelerationy the differenceis one of degree rather than a change of parameters. Energyapplied impulsively to a system is described as shocki energyapplied in a periodic or quasi-periodic manner is calledvibration; and energy applied relatively continuously in onedirection is termed sustained acceleration.

Engineering terms and units used to describeshock include the input or response pulse shape, duration,and amplitude, the latter usually in peak g's. In genera].,flight conditions will be less severe than vehicle environ-ment shock conditions during transportation, handling, orother ground operations. Significant shock environmentwill exist, however, during propulsion stage ignition andseparation. Shock amplitudes as high as 200g have beenrecorded during this sequence. Landing shocks of vehiclesdesigned for soft landings have recorded 17g on the fuse-lage. Shock levels in the foreseeable future are not ex-pected to deviate appreciably from these values.

(3) Vibration

The vibration environment is unique to eachvehicle and cannot be accurately predicted. This envirormentmay be expressed in terms of power spectual density, (4) /cps,which is the square of the root-mean-square accelerationdivided by the nominal bandwidth in cycles per second. Thevalue of this quantity varies from 30() 2 /cps near therocket motor of small vehicles to 1.5(-) 2 /cps for Atlasor Titan class vehicles.

b. Aerodynamics Heating and Internal Heat Sources

The source of high thermal energy are:

Aerodynamic heating during exit andre-entry. Possible skin temperaturesof 1200 to 14001F. Rate of tempera-ture change of 350F/sec can beexpected.

Combustioncf fuels, 3000 to 5000OFor higher

Cryogenic fluids

40

Co Nuclear Radiation From On-Board Sources

Penetrating gamma and neutron radiation

Neutron flux (7.75 x 10 16/sec) permegawatt of operating power

Gamm flux (1.91 x 10 1 7 /sec) permegawatt of operating power

41

SECTION IV

PROPERTIES OF RUBBER AND PLASTIC SEAL MATERIALS

The design of seals necessitates the selection of anappropriate seal material. The physical, cviemical, and thermalproperties of the material as well as the effect of the environ-ment (discussed in Section 111-2) dictate the choice. Thus, itis appropriate to include discussions and tables of materialproperties for both rubbers and plastics which affect the choiceof material for a sealing application. Properties of metals arenot included because the majority of seals encountered in spacestation design are not metallic. Furthermore, the properties ofmetals are well defined and readily obtainable.

i. RUBBER

The problem of selection of elastomers is difficult.Familiar engineering terminology when applied to natural orsynthetic rubbers often has a different meaning than the con-ventional word usage. In order to select a rubber material, itis necessary that the characteristics of the material be under-stood.

Tensile strength, in rubber, refers to the force per unitof original cross section that was applied. A typical stress-strain curve is shown on Figure 18 (Ref. 7 ).

Modulus is the tensile stress that will produce a givenamount of extension. In rubber, the stress-strain relationshipis not proportional. Therefore, the modulus of an elastomerrefers only to a single point on the stress curve.

Hardness is a measure of the resistance of rubber todeformation. The units for hardness are the Durometer numbers.It is measured by pressing a ball or blunt point into the sur-face and measuring the amount of depression.

Resilience is the property of rubber which enables it toreturn to its original shape after having been deformed. Itusually is expressed in percent of the energy returned afterthe removal of the stress which caused deformation,

Hysteresis represents energy lost per loading cycle. Thestress-strain curve of a rubber compound plotted for increasingand decreasing loading conditions shows an energy loss. Thiscan be seen from a typical curve shown on Figure 19. This isgenerally due to the conversion of mechanical energy into heat.

42

4000 j

3500

3000 -arbon BlackReinforcedad

S2500----/-

-2000 -/ -

S1500

1000

Pure Gum0 1 1

0 100 200 300 400 500 600 700 800Elongation, percent

Figure 18 Typical Stream-Strain Curves for SoftVulcanized Natural Rubber (Ref. 7)

10 --

9-

A/7 Z66U S

g54 '

Hysteresis

00 1 2 3 4 5 6 7 8 9 10

Elongation, percent

.. qure 19 Hysteresis Effect of Soft Vulcanized Natural Rubber(Ref. 7 )

43

Permanent set is the deformation of a rubber material thatremains after a given time when a specific load applied for agiven time has been released. Normally, it is specified whetherthe set has resulted from tcnsion, compression, or shear forces.

Stress relaxation refers to the loss in stress when thematerial is held at constant strain over a given time period.

Creep of rubber is the change in strain when the stress isheld constant. Both stress relaxation and creep are expressedas percent change in deformation.

Abrasion resistance refers to the resistance of a rubbercomposition to wear and is measured by the loss of material whenit is brought into contact with a moving abrasive surface.

Tear strength is a measure of the force required to propa-gate a cut in a normal direction to that of the applied stress(the force necessary to initiate tearing).

The shape factor is the change in the compression-deflectionrelationship because of the shape of the part. For pieces havingparallel loading faces and sides normal to these faces, the ratioof face load to the side area provides a measure of the degreeof compressibility. Since rubber is virtually incompressible,the ability of the material to compress depends on the amount ofside area that is free to bulge.

Thermal properties of natural and synthetic rubbers thatare important include coefficient of expansion and the "jouleeffect." The coefficient of expansion varies, depending on thekind and amount of filler added to the gum rubber. The morefiller that is added, the lower the coefficient. The expansionof rubber is approximately one order of magnitude greater thansteel. The "joule effect" is a phenomenon based on the fact thatthe modulus of elasticity of rubber indreases with a rise intemperature. The rubber has to be under strain for this effectto occur. This effect is rather important in an applicationwhere strain and heating may occur. For example, O-rings usedto seal a rotating shaft become heated and attempt to contract.Thus, more heat is generated and eventually results in sealfailure.

Aging describes the deterioration of rubber with the pas-sage of time. The changes which take place in aged rubber canbe described in three ways (Ref. 7).

Disagg~egation resulting in a reduction inaverage molecular weight. This produces a"softening" of the elastomer and is alsoreferred to as chain scission. It is causedby oxidative and/or thermal chain ruptureand depolymerization of polymeric structures.

44

Aggregation resulting in higher molecularweight. This produces a "hardening" of theelastomer. It is caused by cross-linking,branching, cyclization, and further polymeri-zation.

Chemical alteration of the molecule byintroduction of new chemical groups. Thealteration accompanies the foregoing twophenomena but can also be entirely differentin its end result.

Whether the oxidative changes result in a softening orhardening of the composition depends on many factors, some ex-ternal to the material and some internal. These factors can belisted as follows.

External Factors Internal Factors

1. Oxygen generally considered 1. Type of rubberresponsible for most allaging phenomena

2. Heat accelerates the action 2. Degree and type ofof oxygen and at higher tem- vulcanizationperatures produces degradationby itself

3. Pro-oxidants - sulfur, metal 3. Accelerators used andcontaminants their residues

4. Ozone highly active form of 4. Type of compoundingoxygen, producing severe ingredientsdegradation in extremely lowconcentrations

5. Fatigue 5. Processing factors

6. Light and weathering 6. Protective agents

7. Radiation 7. Auto inhibition

Table VI lists the properties of some typical rubber com-pounds affecting their choice as a seal material.

2. PLASTICS

The physica) and mechanical properties of plastic materialsare well defined. Terminology applicable to metallic materialstudies is applicable to plastics with the exception of the factthat elastic deformation occurs during such a small region ofloading that it may be termed nonexistent. The following tables(VII through IX) are of physical, mechanical, and chemical re-sistance properties of some typical plastic materials.

45

Table VIProperties and Applications

of Some Typical Rubber Compounds (Ref. 7)

Sp. t~r. Durometar T.S. Elonge- Max. Min.•[. ~. Aloltr(psi tion 'redr Ataw-....4( ho:ie Hardnea pSvt. SVc. Resit - I?," Range at (percent Tvm.T

N I ddtO- Room at Ro Temp. mp tdncemers (Shore A) Temp) Temp.)

I.yl.1 1.10 40- 500- 100- 300- -20 FaiL100 2200 400 400 0

Poor

I I)UIII( y Iuu0 0.92 30- 1JO0 S01) - 250 -70 Vexy tijud100 700 -60 yood

Veryguod

I, ,*t h1clim 0.85 62- 8000 700 2;-)0 -90 k-xt -v Jup. JtI95 -40 l1 t

VerygoodE~t hy Ilene

Pi ,,pylene

0.85 Ito- 3000 300 S0)0 -50 GuLd • 1xu•t I I ul, I100 -40

Verygoud

i"iuoru-elastomers 1 .4 bO- 2400 350 600 -40 ldir Good

1.95 90 -20Fair

sul fona tedpolyethylene 1.10 30- 2800 500 250 -68 JOu, FxIX*I I leni

95 0Good

t l t 0.91 20- 400o 700 212 -65 Very Excel lent100 40 good

Ve ry

Neoptrene 1.23 20- 40-10 701) 212 h5 L;oud Excellent90 40

I O0 '4 ,, ,|,I -

Piji yVit:t a100 i

4iiI

0.-1 20)- 4000 /%i) 212 -6(4 V.ly l~xi'i. .r'

pr~~~i141~vt I)0.cI ~ ,

PC)~~V: I S1I d

Po.ty ull 4 litiu I . 4.I 2d)- I ¾'" ,I4J 21 2 -1)0J Wi.,, I ,,

Ho -45Veryquod

u;I9) 0.94 .10- 350o 'W00 225 -40 Wki.>d100 -30

Good

Cni Ic.uit" 0.98 214 1500 8o0 550 SlIpe- POOr Pit9 , rior

46

SUPPLEMENTAR

INFORMATION

ERRATA FOR Apim-TR-65-88, PART II

NOTICES

0

When Goverrment drawings, specifications, or other data are used forany purpose other than in connection with a definitely related Goverrment

procurement operation, the United States Government thereby incurs noresponsibility nor any obligation whatsoever; and the fact that theGoverment may have fonnulated, furnished, or in any way supplied the

S said drawings, specifications, or other data, is not to be regarded byimplication or otherwise as in any mariner licensing the holder or anyother person or corporation, or conveying any rights or permivision tomanufacture, use, or sell any patented invention that may in any way berelated thereto.

This document is subject to special export controls and each trans-mittal to foreign goveriments or foreign nationals may be made only withprior approval of the Air Force Flight Dynamics Laboratory (FDTS), Wright-Patterson AFB, Ohio.

Copies of this report should not be returned to the Research and Tech-nology Division unless return is required by security considerations, con.-

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